Wideband, high-fidelity analog optical link design

ABSTRACT

An analog optical link ( 10 ) that provides high-fidelity over bandwidths greater than 1 GHz is presented. The analog optical link ( 10 ) includes a transmitter ( 18 ) having an optical modulator ( 20 ) and a receiver ( 16 ) having an optical demodulator ( 12 ). In one embodiment, the modulator ( 20 ) is a phase modulator ( 20 ) and particularly a pre-emphasis phase modulator ( 20 ) that operates as a frequency modulator at low frequencies. The demodulator ( 12 ) is a frequency demodulator ( 12 ) that includes a feed-forward function for cancelling noise.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U. S. Government has certain rights in this invention pursuant toFAR 52.227-12.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a wideband, high-fidelity analogoptical link, and, more particularly, to a wideband, high-fidelityanalog optical link that includes a pre-emphasis phase modulator in thetransmitter and a feed-forward optical demodulator in the receiver.

2. Discussion of the Related Art

Analog optical links are used in various optical communications systemswhere the transmission of large bandwidth signals are required, withoutthe need for analog-to-digital (A/D) converters or digital-to-analog(D/A) converters. Digitization of analog RF signals is an excellent wayof enabling information transfer, and the capability of digital opticallinks to convey such information is well known. However, digitizing ananalog RF signal having a 5 GHz bandwidth requires approximately a 100Gbps digital link throughput, which is well beyond current data linkcapabilities. Therefore, analog links are required to meet the desiredwideband transmission requirements.

Analog optical links transmit RF signals modulated onto an opticalcarrier signal. The optical carrier signal generally is transmittedalong a fiber optic cable or through free space to a receiver where itis demodulated to recover the RF data. The optical link allows the RFdata to be transmitted with low losses and at high bandwidths, and thusis attractive in many communications systems to provide the desiredperformance, especially high frequency RF communications systems thattransmit signals in the GHz bandwidth range. Also, telescopes used totransmit optical signals in free space have a much greater directivitythan RF antennas of comparable size.

The analog optical links being discussed herein need to be widebandanalog signals having high-fidelity. A wideband signal discussed hereinmay be up to tens of GHz. By high fidelity, it is meant transmission ofsignal information with resultant dynamic range and a signal-to-noise(SNR) equivalent to that achievable if the signal were digitized withmore than six bits and transmitted using a digital communication link.To have the desired performance for various communications systems, theoptical link must provide a good dynamic range, i.e., allow thesimultaneous transmission of signals having widely varying amplitudesthat do not interfere with each other, with minimal optical powerrequirements.

Currently, intensity modulation (IM) is the dominant optical modulationchoice for analog optical links. In IM, the intensity of the opticallight is modulated with the RF signal. Unfortunately, IM does notprovide high enough performance because significant transmission poweris required to provide the desirable dynamic range and signal-to-noiseratio (SNR) for a particular application. In fact, ideal linear IMrequires 9 dB more received optical power than ideal suppressed carrieramplitude modulation (AM) to get the same demodulated SNR. To overcomethis problem, known intensity modulation optical links provide a seriesof optical amplifiers to boost the optical carrier signal power as itpropagates along an optical fiber. The number of optical amplifiersneeded can be costly. Also this technique cannot be used forlong-distance free space links.

Wideband frequency modulated (FM) or phase modulated (PM) optical linkscan theoretically use the extremely wide bandwidth available at opticalfrequencies to achieve much better dynamic range and SNR than IM opticallinks for the same received power. For example, phase modulation havinga peak phase deviation of 10 radians has a 26 dB greater link SNRpotential compared to ideal IM, and a 17 dB greater SNR potential thansuppressed-carrier AM.

Known FM or PM communications systems must significantly modulate thecarrier frequency or phase to achieve better dynamic range and SNRperformance than AM. In other words, the frequency deviation or phasedeviation of the carrier signal which is induced by the RF input signalmust be large enough to increase the bandwidth of the modulated carriersubstantially beyond that of an AM modulated carrier.

Phase modulated optical links generate an additive noise floor that ishigher at lower frequencies. Therefore, the sensitivity of multi-octavefrequency ranges is degraded at the lower frequencies. Also, sinceoptical frequency demodulators are generally used to demodulate a phasemodulated signal, an RF integrator is needed to recover the signal.Multi-octave RF integrators generally have a large gain slope across thesignal frequency range and can be difficult to implement.

Frequency modulated optical links do not have the low frequency noiseproblem that phase modulated optical links have. However, directwideband frequency modulation of an optical beam is much more difficultand less desirable than external phase modulation. Direct frequencymodulation of the carrier wave laser source requires the laser beam tobe co-located with the RF input signal so that no photonic remoting ofthe laser beam is allowed. Direct frequency modulation can alsointerfere with line width reduction circuitry, which is important tomaintain the low overall FM or PM link noise floor. External frequencymodulation using an RF integrator followed by an external phasemodulator has the same gain slope and implementation problems associatedwith the phase demodulator.

What is needed is a wideband analog optical link design which combinesthe low frequency performance of a frequency modulated link with thesimplicity of a phase modulated link. It is therefore an object of thepresent invention to provide such a link.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, an analogoptical link is disclosed that provides high-fidelity over bandwidthsgreater than 1 GHz. The analog optical link includes a transmitterhaving an optical modulator and a receiver having an opticaldemodulator. In one embodiment, the modulator is a phase modulator, andparticularly a pre-emphasis phase modulator that operates as a frequencymodulator at low frequencies. Also, in one embodiment, the demodulatoris a frequency demodulator that includes a feed-forward function forcancelling noise.

Additional objects, features and advantages of the present inventionwill become apparent from the following description and appended claimstaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of wide-band, high-fidelity analogoptical link, according to an embodiment of the present invention;

FIG. 2 is a plan view of an optical phase modulator used in thetransmitter of the optical link shown in FIG. 1;

FIG. 3 is a graph with effective electrode length on the vertical axisand RF frequency on the horizontal axis depicting the performance of theoptical phase modulator shown on FIG. 2;

FIG. 4 is a graph with sensitivity on the vertical axis and RF frequencyon the horizontal axis depicting the performance of the optical phasemodulator shown in FIG. 2;

FIG. 5 is a schematic block diagram of a feed-forward optical PMdemodulator used in the receiver of the optical link shown in FIG. 1,according to an embodiment of the present invention;

FIG. 6 is a schematic block diagram of a feed-forward optical FMdemodulator that can be used in the receiver of the optical link shownin FIG. 1, according to another embodiment of the present invention;

FIG. 7 is a schematic block diagram of a feed-forward optical PMdemodulator, including an embedded phase modulator, that can be used inthe receiver of the optical link shown in FIG. 1, according to anotherembodiment of the present invention; and

FIG. 8 is a schematic block diagram of a feed-forward optical FMdemodulator, including an embedded phase modulator, that can be used inthe receiver of the optical links shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following discussion of the preferred embodiments directed to apre-emphasis phase modulated analog optical link is merely exemplary innature, and is in no way intended to limit the invention or itsapplications or uses.

FIG. 1 is a schematic block diagram of a communications system 10,including a transmitter 18 and a receiver 16, according to an embodimentof the present invention. According to the invention, an RF signal s(t)to be transmitted is a wideband high-fidelity analog signal. For thediscussion herein, wideband is greater than 1 GHz, and particularlygreater than 4 GHz. Further, high-fidelity is defined as a signal havingthe resultant dynamic range and SNR equivalent to that achievable if thesignal were digitized with more than 6 bits and transmitted using adigital communication link.

The RF input signal s(t) to be transmitted on a lossy analog opticalchannel 14 is applied to an optical modulator 20 within the transmitter18. The channel 14 can be either free space or a fiber optic cable. Theoptical modulator 20 can be either a phase modulator or a frequencymodulator within the scope of the present invention, as will bediscussed below. An optical carrier signal from a laser source 22 isalso applied to the modulator 20. In one design, the RF signal s(t)generates an electric field across an optical waveguide in the modulator20 through which the carrier signal is propagating. Changes in theelectric field cause the index of refraction of the waveguide in themodulator 20 to change. This causes the carrier signal to speed up orslow down in association with changes in the RF signal, thus modulatingthe carrier signal. This is one example of modulation of an RF signalknown in the art. Other modulation schemes that provide phase modulationor frequency modulation can also be employed within the scope of thepresent invention. The modulated carrier signal is amplified by anoptical amplifier 24 and then transmitted on the lossy optical channel14.

The optical signal on the channel 14 is applied to an optical low-noiseamplifier (LNA) 26 in the receiver 16. The amplified optical signal isthen bandpass filtered by an optical filter 26 that removes some of thenoise generated by the optical amplifiers 24 and 26 and the lossychannel 14. An optical limiter 46 receives the amplified and filteredoptical signal, and acts to suppress amplitude noise from the opticalamplifiers 24 and 26, the laser source 22 and any time-varying linklosses. The optical signal is then applied to a feed-forward opticaldemodulation system 12 that demodulates the optical signal to remove theRF signal. Various embodiments of the optical demodulation system 12will be discussed below.

According to one embodiment of the present invention, the modulator 20is a phase modulator designed to operate at a wideband frequency rangewithout significant noise degradation at lower frequencies.

FIG. 2 is a plan view of the phase modulator 20 removed from thetransmitter 18. The phase modulator 20 includes a semiconductor opticalwaveguide 120 and opposing RF electrodes 122 and 124 formed within thewaveguide 120, as shown. In one embodiment, the optical waveguide 120 isa lithium niobate material, but can be any suitable optical waveguidematerial or architecture known in the art. The RF input signal isapplied to the electrode 122 that creates an electric field across thewaveguide 120. The electric field in the waveguide 120 changes the indexof refraction of the waveguide 120, that affects the propagation speedof an optical carrier signal 126 from the laser 22 propagating down thewaveguide 120. Therefore, the carrier signal 126 is modulated by the RFinput signal.

According to the invention, the electrodes 122 and 124 are relativelylong velocity-matched electrodes that provide the RF loss versus RFfrequency needed to produce the desired Vπ versus RF frequency over theentire wide bandwidth. Vπ identifies the amount of voltage applied tothe electrodes 122 and 124 that causes a phase shift in the opticalcarrier signal 126 of π radians. Phase modulators with lower Vπ at lowerfrequencies will emphasize the lower frequencies and improve thelow-frequency fidelity of multi-octave optical links. The frequency ofthe RF input signal s(t) determines whether the modulator 20 is a phasemodulator or a frequency modulator. The difference in the lossesencountered by the RF signal at different frequencies causes themodulator 20 to act like a frequency modulator at lower frequencies. Inother words, the losses in the modulator 20 mimic the effects of anintegrator that causes the phase modulator 20 to act like a frequencymodulator at low frequencies. The operation of the modulator 20 in thismanner provides a pre-emphasis at the lower frequencies.

The RF losses in velocity-matched phase modulators generally increasewith RF frequency. These RF losses are small dB per unit length ofelectrode and are generally proportional to f^(x), where ½<×<1. Thisimplies that the lower RF frequencies will have longer effectiveinteraction lengths if the phase modulator 20 is made long enough.Longer interaction lengths give the RF signal more time to modulate theoptical carrier signal 126, and thus give lower Vπ. The lower Vπ at thelower end of the frequency range boosts low frequency signals above theexcess link noise generated in the demodulation system 12. FIG. 3 is agraph with effective electrode length on the vertical axis and RFfrequency on the horizontal axis, and FIG. 4 is a graph with sensitivity(Vπ) on the vertical axis and RF frequency on the horizontal axis thatshows this relationship.

Normally, phase modulators are not made long enough to see much of thisVπ frequency dependency. These modulators are typically designed byreducing the Vπ at the upper end of the frequency range and generallyproviding Vπ roughly independent of frequency. Therefore, the knownphase modulators are not made any longer than necessary, until theelectrode losses exceed roughly 6 dB at the highest frequency.

As discussed herein, by making the electrodes 122 and 124 longer thanthe corresponding electrodes of the known phase modulators of this type,the modulator 20 operates as a hybrid phase and frequency modulator.This is because the RF losses in the electrodes 122 and 124 aredifferent at different RF frequencies, and thus the amount of modulationis different for the different RF frequencies. In the known modulatorsof this type, the electrodes were typically made short so that themodulation of the carrier signal was substantially linear over theentire operational bandwidth of the modulator. In other words, the knownmodulators were designed so that the same amount of phase modulationoccurred for all of the frequencies over the operational range. Incontrast, the modulator 20 of the present invention, operates over amuch wider frequency band, and takes advantage of the RF losses in theelectrodes 122 and 124 to provide the desired performance. In otherwords, the fact that the modulation is different for different RFfrequencies improves the fidelity of the system 10 over the entire widebandwidth.

In one embodiment, a relatively long electrode has 18 dB of electricalloss at 18 GHz and 6 dB of electrical loss at 2 GHz. So, the effectiveelectrode length at 2 GHz is approximately three times as long as theeffective electrode length at 18 GHz. Thus, the phase modulation depth(phase shift of the optical carrier) is approximately three times asmuch for a signal at 2 GHz as a signal at 18 GHz with the same powerlevel. At the phase demodulator output, a signal at 2 GHz will be 20log(3) or 9.54 dB higher than a signal at 18 GHz with the same inputpower level. Thus, even if the link-generated output noise floor is 9 dBhigher at 2 GHz compared to the 18 GHz, the length will not degrade theSNR of the signal at 2 GHz anymore than the signal at 18 GHz. Thus, thedesign of the phase modulator 20 accommodates a bandwidth between 2-18GHz for the same SNR.

The RF electrodes 122 and 124 and the length of the waveguide 120 can bedesigned to achieve the pre-emphasis desired for the particularfrequency range and sensitivity requirements. Since optical phasedemodulation will most likely use a frequency demodulator, this lowfrequency pre-emphasis in the phase modulator 20 will reduce oreliminate the need for further equalization to provide a flat link RFgain across the frequency range. Thus, in the demodulator system 12, theRF integrator is replaced with a simple passive 90° hybrid and possiblyan equalizer.

This pre-emphasis makes a PM optical link look more like an FM opticallink. If the electrode losses in dB are proportional to the RF frequencyand the waveguide 120 is long enough, then the magnitude response of thelink is just like an FM link. If the electrode losses in dB areproportional to the square root of the RF frequency and the waveguide120 is long enough, then the magnitude response of the link is mid-waybetween FM and PM.

FIG. 5 is a schematic block diagram of the demodulation system 12,according to an embodiment of the present invention. In this example,the demodulation system 12 is a feed-forward optical PM demodulationsystem. The filtered optical signal from the limiter 46 is applied to anoptical splifter 30 within the demodulation system 12 that provides twosplit optical signals that are copies of the modulated optical carriersignal. The splitter 30 does not have to be a 50/50 splitter, but can beany relative power splitter suitable for a particular application, aswould be understood to those skilled in the art.

One of the signals from the splitter 30 is applied to an unbalancedoptical MZI 32 within a coarse PM demodulator 34. As is known in theart, an unbalanced optical MZI separates an input optical signal intotwo optical paths, where the two path lengths are different so onesignal is delayed relative to the other signal. The two signals are thencombined in a directional coupler in the MZI 32 to produce two outputsignals. The MZI 32 translates frequency modulation into intensitymodulation. And the two outputs are complementary of each other. Inother words as the frequency of the optical carrier increases theintensity of one output goes up while the intensity of the other outputgoes down. The relative delay is a design parameter, but it should beset to a quadrature bias point. The MZI is quadrature biased when therelative delay is a multiple of the period of the unmodulated opticalcarrier offset by a quarter of its period. When the MZI is quadraturebiased, the two outputs are most linear and have a common nominalintensity. The operation of an unbalanced Mach-Zehnder interferometerincorporating a directional coupler is well known to those skilled inthe art.

One of the outputs from the MZI 32 is applied to a first photodetector36 and the other output from the MZI 32 is applied to a secondphotodetector 38 that demodulate the optical signals to generaterepresentative electrical signals. The two electrical signals are thenapplied as inputs to a differential amplifier 40. The differentialamplifier 40 amplifies the difference between the two complementary RFsignals, and cancels or nulls the bias as well as all common mode noiseand distortion. The combination of the photodetectors 36 and 38 and thedifferential amplifier 40 make up a balanced photoreceiver 42. Thebalanced photoreceiver 42 strips away the optical carrier from theoptical signal, and provides a frequency demodulated signal at theoutput of the differential amplifier 40. The output of the differentialamplifier 40 is then applied to an RF integrator 44 to provide the PMdemodulation. Using an unbalanced MZI and photoreceiver as discussedherein is a known technique for providing frequency demodulation.

The RF output from the coarse demodulator 34 is a coarse representationof the RF signal s(t), and is defined here as A[s(t)−ε(t)]. In thisequation, A is the amplitude scale factor between the RF signal s(t) andthe coarse demodulator output, and the error signal ε(t) represents theerror in the coarse demodulator output consisting of noise and signaldistortion caused by the demodulation process. The PM theoreticalminimum noise is from amplified spontaneous emissions (ASE) noise fromthe optical amplifiers 15 and 26 beating against an unmodulated opticalcarrier signal (carrier×ASE noise). In a properly designed link, othernoise sources such as shot noise from photodetectors 36 and 38, thermalnoise from the differential amplifier 40, phase and relative intensitynoise (RIN) from the laser source 22, and ASE×ASE noise are smallcompared to the carrier×ASE noise.

For small phase deviations, the noise generated in the coarsedemodulator 34 is near the theoretical PM value. But as the phasedeviations in the carrier signal approach or exceed π radians, the noisegenerated in the coarse demodulator 34 rises substantially above thetheoretical value and the third-order distortion can also becomesignificant. Thus the error ε(t) in the coarse demodulator outputbecomes relatively large. In other words, when the phase modulator 20modulates the carrier signal with the RF signal in such a way as toproduce large phase deviations, the performance of the coarsedemodulator 34 degrades. Recall that in order to provide substantiallyimproved dynamic range and SNR performance compared to IM links, PMlinks must have large phase deviations. Therefore, the demodulator 34alone cannot provide increased performance beyond the traditional IMformat. Additionally, the demodulator 34 has poor linearity, similar tothe known IM links that use a quadrature-bias Mach-Zehnderinterferometer in the transmitter end for modulation and a photodetectorin the receiver end for demodulation. Therefore, the linearity of thedemodulator 34 is also not a significant improvement over the state ofthe art. More signal processing is thus required to provide the desiredperformance.

The signal from the demodulator 34 is applied to an RF power divider 48that splits the signal into two copies of itself. One of the signals isapplied to an inverter 50 that inverts the signal to be −A[s(t)−ε(t)].This signal is applied to an RF amplifier 52 that amplifies the invertedsignal back to a level near that of the RF signal at the input to thephase modulator 20 in the transmitter 18. Thus the output of the RFamplifier 52 is −[s(t)−ε(t)]. In an alternate embodiment, the powerdivider 48 and the inverter 50 can be combined as a single 180° hybridthat provides an inverted and a non-inverted copy of an input signal.Other components can also be used to provide a split signal and aninverted signal from the coarse demodulator 34, as would be appreciatedby those skilled in the art. Additionally, the amplifier 52 can bepositioned at other locations in the demodulation system 12 other thanafter the inverter 50, as would also be appreciated by those skilled inthe art.

The other split signal from the splitter 30 is applied to an opticaldelay device 54 that delays the signal a period of time relative to thepropagation time of the demodulator 34 and other components. The delaydevice 54 provides a timing alignment between the split optical signaland the signal from the amplifier 52. The delayed optical signal withphase modulation s(t) from the delay device 54 and the signal−[s(t)−ε(t)] from the amplifier 52 are aligned with each other in time.The phase modulator 56 operates in the same manner as the phasemodulator 20, where the optical signal with phase modulation s(t) ismodulated by the RF signal −[s(t)−ε(t)] to get an optical signal withphase modulation ε(t). In other words, the modulated carrier signal fromthe delay device 54 is again modulated in the phase modulator 56 by anRF signal that includes −s(t). Therefore, the signals s(t) and −s(t)cancel, leaving an optical carrier signal that is phase modulated withε(t), i.e., the additive inverse of the noise and distortion in thesignal from the demodulator 34.

The purpose of the phase modulator 56 is to “null” or suppress theoriginal modulation of the optical carrier using the output of thecoarse demodulator 34 as an estimate of the original modulation. This“nulling” effectively reduces the phase modulation from wideband tonarrow band, which drastically reduces the modulation bandwidth. Theresulting optical signal can then be filtered to a much narrowerbandwidth than the received transmission bandwidth affecting theresidual modulation on the optical carrier.

The optical carrier signal from the phase modulator 56 is then appliedto a narrow band optical filter 58. The purpose of the narrowbandoptical filter 58, is to reduce the bandwidth of the optical noise,thereby reducing the optical nose power and increasing the opticalcarrier-to-noise ratio (CNR) at the input to the fine PM demodulator 60.

The filtered optical carrier signal from the filter 58 is applied to afine PM demodulator 60 that demodulates this signal in the same manneras the demodulator 34. Particularly, the demodulator 60 includes anunbalanced MZI 62 that generates complementary intensity modulatedoutput signals that are applied to photodetectors 64 and 66 todemodulate the signals. The electrical complementary signals from thephotodetectors 64 and 66 are applied to a differential amplifier 68 thatgenerates a difference output signal that removes the bias as well asall common mode noise and distortion. The combination of thephotodetectors 64 and 66 and the amplifier 68 make up a balancedphotoreceiver 70. This signal is then applied to an RF integrator 72 togenerate the phase demodulated RF error signal ε(t). Although the errorsignal ε(t) is large compared to the noise level of an idealdemodulator, it is small compared to the signal s(t) in a properlydesign link above threshold. Thus the phase deviations in the carriersignal at the input to the fine demodulator 60 are small, and thereforethe noise generated in the fine demodulator 60 is near the theoreticalPM value. The small phase deviations also allow the fine demodulator 60to demodulate the error signal ε(t) with very little distortion.

The signal A[s(t)−ε(t)] from the power divider 48 is applied to an RFdelay device 76 to align it in time with the signal from the finedemodulator 60. The signal from the RF delay device 76 is applied to anamplitude adjust device 78 to remove the scale factor A to generate thesignal s(t)−ε(t). This signal is applied to a power combiner 80 alongwith the error signal ε(t) from the demodulator 60, that when combinedprovides a demodulated output of the RF signal s(t) with very littledistortion and an additive noise level near the theoretical PM value.Therefore, the operation of the demodulator 12 provides a substantiallyclean copy of the RF input signal applied to the amplifier 18.

The filter 58 allows the fine demodulator 60 to have a much higher inputoptical CNR, and thus, will introduce much less excess noise due tooptical noise beating against itself (noise×noise). The coarsedemodulator 34 will still produce excess noise from optical noise×noisebut it will be suppressed by the feed forward noise cancellation throughthe two paths from the coarse modulator output to the full feed forwardoutput. Thus, the filter 58 effectively lowers the demodulator'sthreshold CNR. For phase modulation, the noise floor reduction isgreatest at the low end of a multi-octave frequency range.

The same demodulation process as discussed above for the communicationssystem 10 can be used for those systems that provide frequencymodulation, as opposed to phase modulation, of the RF input signal ontothe optical carrier wave 126. FIG. 6 is a schematic block diagram of afeed-forward optical FM demodulation system 90 depicting this variation.The transmitter portion and the optical link portion of thecommunications system 10 are not shown in this embodiment. Thetransmitter 18 would include any suitable frequency modulation device tofrequency modulate the optical carrier signal from the laser source 22with the RF input signal s(t). In one embodiment, an integrator isemployed in combination with the phase modulator 20 to provide frequencymodulation, as is well understood in the art.

The frequency modulated optical carrier signal is applied to the opticalsplitter 30 in the demodulation system 90 in the same manner asdiscussed above. The same reference numerals in the demodulation system90 represent the same components as in the demodulation system 12, andoperate in the same manner. In this embodiment, the coarse PMdemodulator 34 is replaced with a coarse FM demodulator 92, and the finePM demodulator 60 is replaced with a fine FM demodulator 94. Thedemodulators 34 and 60 become frequency demodulators by removing the RFintegrators 44 and 72, as shown. Additionally, the amplifier 52 isreplaced with an RF integrator 96 so that the RF signal from theinverter 50 is frequency modulated by the combination of the integrator96 and the phase modulator 56. The resulting cancellation of the errorsignal is accomplished in the same manner as discussed above.

FIG. 7 is a schematic block diagram of another feed-forward optical PMdemodulation system 100 that is an alternative embodiment to the PMdemodulation system 12, discussed above. The demodulation system 100uses the coarse FM demodulator 92 and a fine FM demodulator 102 insteadof the coarse and fine PM demodulators 34 and 60 because thedemodulators 92 and 102 do not include the RF integrators 44 and 72 inthis design. Additionally, the phase modulator 56 has been removed, andreplaced with a phase modulator 106 positioned within an optical path ofan unbalanced MZI 104 in the fine demodulator 102. By positioning thephase modulator 106 in the MZI 104 instead of before the finedemodulator 102, the modulation on the optical carrier signal can becancelled in the fine demodulator 102 without the need for an RFintegrator 44 or 96 preceding the phase modulator 106. The modulation isnot cancelled directly. Instead the MZI bias is rapidly adjusted to nullthe modulation in the frequency-to-intensity conversion that occurs inthe directional coupler at the output of the MZI 104. This rapid biasadjustment keeps the MZI very near quadrature all the time, which allowsthe fine demodulator 102 to demodulate the error signal e(t) with verylittle added noise and distortion as in the other embodiments discussedabove. In this embodiment, an RF integrator 108 receives an output fromthe power combiner 80 to provide the phase demodulation.

FIG. 8 is a schematic block diagram of another feed-forward optical FMdemodulator system 114 that employs the phase modulator 106 in the pathof the unbalanced MZI 104 in the fine FM demodulator 102. This design isfor FM demodulation because the RF integrator 108 has been eliminated.

The use of unbalanced MZIs, photodetectors and differential amplifiersin the various coarse and fine demodulators discussed above is by way ofa non-limiting example. Other designs, within the scope of the presentinvention, may include other types of demodulators that would operatesubstantially in the same manner as discussed herein.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. An analog optical system for transmitting ananalog optical signal on an optical channel, said optical signal havinga bandwidth greater than 1 GHz, said system comprising: a transmitterfor transmitting the optical signal, said transmitter including anoptical modulator responsive to a carrier signal and an RF input signal,said modulator modulating the carrier signal with the RF signal togenerate the transmitted optical signal, said modulator supporting thebandwidth of the optical signal and having high-fidelity, wherein themodulator includes an optical waveguide and opposing electrodes, wherethe opposing electrodes having a length such that the modulator operatesas a frequency modulator for an RF signal having a first frequency andas a phase modulator for an RF signal having a second frequency, thesecond frequency being higher than the first frequency; and a receiverreceiving the optical signal from the optical channel, said receiverincluding an optical demodulator, said demodulator demodulating theoptical signal to remove the RF signal from the carrier signal saiddemodulator supporting the bandwidth of the optical signal and havinghigh-fidelity.
 2. The system according to claim 1 wherein the modulatoris selected from the group consisting of phase modulators and frequencymodulators.
 3. The system according to claim 1 wherein the demodulatoris selected from the group consisting of phase demodulators andfrequency demodulators.
 4. The system according to claim 1 wherein themodulator is a phase modulator and the demodulator is a frequencydemodulator.
 5. The system according to claim 1 wherein the transmitterincludes an amplifier for amplifying the optical signal prior to beingtransmitted.
 6. The system according to claim 1 wherein the receiverincludes a low noise amplifier, an optical filter and an opticallimiter.
 7. The system according to claim 1 wherein the modulator is apre-emphasis phase modulator.
 8. The system according to claim 7 whereinthe phase modulator includes an optical waveguide and opposingelectrodes, where the electrodes are long enough to provide an RF lossversus RF frequency that provides an optimized Vπ for a frequencybandwidth greater than 1 GHz.
 9. The system according to claim 1 whereinthe demodulator is a feed-forward frequency demodulator.
 10. The systemaccording to claim 9 wherein the demodulator includes a coarse opticaldemodulation device responsive to the optical signal, said coarsedemodulation device generating a coarse RF signal including the RF inputsignal and an additive inverse of an error RF signal, said demodulatorfurther including a fine optical demodulation device demodulating theerror RF signal and generating the error RF signal, said demodulatorfurther including a combiner combining the coarse RF signal and theerror signal from the fine demodulation device.
 11. A wideband,high-fidelity analog optical system for transmitting an analog opticalsignal on an optical channel, said optical signal having a bandwidthgreater than 1 GHz, said system comprising: a transmitter fortransmitting the optical signal, said transmitter including apre-emphasis phase optical modulator responsive to a carrier signal andan RF input signal, said modulator modulating the carrier signal withthe RF signal to generate the transmitted optical signal, said modulatorsupporting the bandwidth of the optical signal and having high-fidelity;and a receiver receiving the optical signal from the optical channel,said receiver including a feed-forward optical frequency demodulator,said demodulator demodulating the optical signal to remove the RF signalfrom the carrier signal, said demodulator supporting the bandwidth ofthe optical signal and having high-fidelity.
 12. The system according toclaim 11 wherein the transmitter includes an amplifier for amplifyingthe optical signal prior to being transmitted and wherein the receiverincludes a low noise amplifier, an optical filter and an opticallimiter.
 13. The system according to claim 11 wherein the phasemodulator includes an optical waveguide and opposing electrodes, wherethe electrodes are long enough to provide an RF loss versus RF frequencythat provides an optimized Vπ for a frequency bandwidth greater than 1GHz.
 14. The system according to claim 11 wherein the modulator includesan optical waveguide and opposing electrodes, where the opposingelectrodes are long enough so that the modulator acts as a phasemodulator at high frequency and the frequency modulator at lowfrequency.
 15. The system according to claim 11 wherein the demodulatordevice includes a coarse optical demodulation device responsive to theoptical signal, said coarse demodulation device generating a coarse RFsignal including the RF input signal and an additive inverse of an errorRF signal, said demodulator further including a fine opticaldemodulation device responsive to a modulated error RF signal, said finedemodulation device demodulating the error RF signal and generating theerror RF signal, said demodulator further including a combiner combiningthe coarse RF signal and the error signal from the fine demodulationdevice.
 16. A method of transmitting an analog optical signal on anoptical channel over a bandwidth greater than 1 GHz, said methodcomprising the steps of: modulating a carrier signal with an RF inputsignal to generate the transmitted analog optical signal, said step ofmodulating providing high-fidelity over the entire bandwidth of theoptical signal, wherein the modulator includes an optical waveguide andopposing electrodes, where the opposing electrodes having a length suchthat the modulator operates as a frequency modulator for an RF signalhaving a first frequency and as a phase modulator for an RF signalhaving a second frequency, the second frequency being higher than thefirst frequency; and demodulating the analog signal to remove the RFsignal from the carrier signal, said step of demodulating providinghigh-fidelity over the entire bandwidth of the optical signal.
 17. Themethod according to claim 16 wherein the step of modulating includesphase modulating the carrier signal.
 18. The method according to claim16 wherein the step of demodulating includes frequency demodulating thecarrier signal.